Ever since I started working on gamma handling, I had been trying to
concoct a scene that clearly and unambiguously demonstrates why anything
other than "assumed_gamma 1.0" is doing it wrong. I /knew/ this was the
case, but somehow it eluded all my attempts at nailing it down.

Until I stumbled across the description of the `exponent` reflection
parameter in the docs: Here was a feature that probably had been
invented for the sole purpose of working around a phenomenon that early
POVers failed to understand, but which to me shouts "Gamma!" at the top
of its lungs.

So inspired by that text, here it is now at last: The smoking gun of
gamma handling.


The first image shows the scene as it should be, rendered with
`assumed_gamma 1.0` and proper colour math. What you see is a somewhat
reflective checkered plane, a set of somewhat reflective spheres with
highlights, and a black background. That's all, nothing else fancy in
there. (I even turned off ambient and radiosity, as it would have made
the next step far more difficult.)


The second image shows essentially the same scene with `assumed_gamma
2.2`, with the diffuse settings (`diffuse` and `brilliance`) adjusted to
get exactly the same diffuse effect out of the different colour math.
Nothing else changed whatsoever.

I'd like to draw your attention to the following details:

Exhibit A: The highlights. On the darker spheres they get dimmer, while
staying at about the same brightness (actually even getting a bit
brighter) on the bright spheres.

Exhibit B: The reflections. In the shadows and darker portions they seem
to disappear almost /completely/, while in the brightly lit portions
they even get brighter.

Exhibit C: The terminators. In the reflections the transition between
the illuminated and shadowed sides of the spheres become excessively sharp.


Just to scare your kids I've also attached a the third image, rendered
with "assumed_gamma 5.0" to exaggerate the effect. If the "assumed_gamma
2.2" image did not convince you, this one should: All the flaws visible
here are due to non-linear colour math, and while they are a tad more
subtle with a gamma of 2.2, they're still present.


I rest my case.

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Attachments:
Download 'smoking_gamma_1.0.png' (173 KB) Download 'smoking_gamma_2.2.png' (159 KB) Download 'smoking_gamma_5.0.png' (153 KB)

Preview of image 'smoking_gamma_1.0.png'

Preview of image 'smoking_gamma_2.2.png'

Preview of image 'smoking_gamma_5.0.png'